Agronomic and environmental consequences of using liquid mineral concentrates on arable farms

In regions with intensive livestock systems, the processing of manure into liquid mineral concentrates is seen as an option to increase the nutrient use efficiency of manures. The agricultural sector anticipates that these products may in future be regarded as regular mineral fertilisers. We assessed the agronomic suitability and impact on greenhouse gas (GHG) and ammonia emissions of using liquid mineral concentrates on arable farms.

The phosphate requirements on arable farms were largely met by raw pig slurry, given its large regional availability. After the initial nutrient input by means of pig slurry, the nitrogen/phosphate ratio of the remaining nutrient crop requirements determined the additional amount of liquid mineral concentrates that can be used. For sandy soils, liquid mineral concentrates could supply 50% of the nitrogen requirement, whereas for clay soils the concentrates did not meet the required nitrogen/phosphate ratio. The total GHG emissions per kg of plant available nitrogen ranged from -65 to 33 kg CO2 -equivalents. It increased in the order digestates < mineral fertiliser < raw slurries.

Liquid mineral concentrates had limited added value for arable farms. For an increased suitability it is necessary that liquid mineral concentrates do not contain phosphate and that the nitrogen availability is increased. In the manure-processing chain, anaerobic digestion had a dominant and beneficial effect on GHG emissions.